1. Field of the Invention
The present invention relates to an AM receiver in which the bandwidth of an intermediate frequency band amplifier is equivalently variable.
2. Description of the Prior Art
Conventionally, in AM receivers, there are those which are constituted in such a manner that the bandwidth of an intermediate frequency (IF) amplifier can be switched between the wide bandwidth and the narrow bandwidth. For example as shown in FIG. 1, the AM receiver of this kind is provided with a wide bandwidth intermediate frequency (IF) amplifier 4 and a narrow bandwidth intermediate frequency (IF) amplifier 5 and is constituted in such a manner that an output of a radio frequency (RF) amplifier 1 is supplied to a frequency converter consisting of a mixing circuit 2 and a local oscillator 3 and is converted into an intermediate frequency signal; this intermediate frequency signal is supplied to the wide bandwidth IF amplifier 4; an output of the wide bandwidth IF amplifier 4 is supplied to a detector 6 through the narrow bandwidth IF amplifier 5 and is detected therein; the output of the wide bandwidth IF amplifier 4 is also directly supplied to a detector 7 and is detected therein; and either one of a detection output from the detector 6 and a detection output from the detector 7 is selected by selector means 8.
As described above, according to the conventional AM receiver, the wide and narrow bandwidths can be changed over by switching the selector means 8; however, it has such a drawback that it is impossible to select a bandwidth between the wide and narrow bandwidths.
In addition, as shown in FIG. 2, there is an AM receiver provided with selector means 8A which is switched in response to an external signal in place of the selector means 8, a DC component detecting circuit 11 to detect the DC component level in the output of the detector 6, and a comparator 12 to detect whether or not an output of the DC component detecting circuit 11 is not lower than a predetermined value, thereby switching the selector means 8A responsive to an output of the comparator 12.
In such an AM receiver as shown in FIG. 2, when the received signal strength is small, the narrow bandwidth is automatically selected, while when the received signal strength is large, the wide bandwidth is automatically selected. However, there is such a drawback that it is impossible to select a bandwidth between the narrow bandwidth and the wide bandwidth in accordance with the received signal strength.